How we check the roofing figures

What each calculator does with the numbers you type, where every starting figure comes from, how often we re-read those sources, what the results cannot tell you, and how to tell us when something is wrong.

The short version

• Roof area is geometry: the plan of each section times a factor that depends only on the pitch. No estimate, no averaging.
• Everything you buy (bundles, rolls, tiles, battens, ridge pieces) is worked out from a coverage figure printed by a manufacturer or a merchant, then rounded up to whole units.
• Every starting figure is shown next to its field with its source and the day we read it, and you can overwrite any of them.
• Guides and charts on this site are produced by the same code as the calculators, so a chart and the calculator never disagree.

Roof area and pitch

You describe the roof as one or more rectangles seen from above: the length along the ridge, the width from eave to eave, the overhang past the walls and the pitch. An L-shaped house is two sections; a garage or a dormer is another. The overhang is added on every side before anything else is computed.

Pitch is entered as rise per 12 of run (x:12), in degrees, or as a rise and a run measured on site. Degrees = arctan(rise / 12); the area factor = √(1 + (rise / 12)²), which is the same thing as 1 / cos(pitch). A 6:12 roof is 26.6° and multiplies the plan by 1.118. The roof pitch calculator shows all four forms side by side and accepts pitches up to 24:12.

Roof area = plan area x the pitch factor. When every slope of a section has the same pitch, this is exact for a gable and for a hip roof alike; the shape only changes the lengths of the edges. A gable has a ridge as long as the section and two sloping rakes at the ends. A hip roof has a ridge of length minus width and four hips, each (width / 2) x √(2 + (rise / 12)²) long. Those lengths drive the ridge pieces and the drip edge.

Shingles (US)

• Squares = roof area / 100 sq ft. Squares to order add waste: 10% on a simple gable and 15% with hips or valleys, the two ends of the range IKO recommends.
• Bundles = squares to order x bundles per square, rounded up: 3 for both 3-tab and architectural shingles in the Owens Corning specification guide. Heavier designer shingles take 4 or 5; the field is there to change.
• Underlayment rolls = squares to order / squares per roll, rounded up: 10 squares for a synthetic roll, 4 for #15 felt.
• Ridge cap bundles = (ridge + hips) / the linear feet one bundle covers, rounded up: 25 ft or 29.53 ft from the two products listed.
• Nails = shingles per square (80 for 3-tab, 64 for architectural) x squares as laid x 4 nails per shingle, then / 185 nails per pound for 1-1/2 in roofing nails.
• Drip edge pieces = (eaves + rakes) / 10 ft per piece, rounded up.
• Cost = squares of roof x the installed price per square for the material, low and high, from the HomeAdvisor cost guide (published per sq ft, multiplied by 100).

Calculator: roof shingle calculator.

Roof tiles and battens (UK)

• Tiles = roof area in m² x tiles per m² x (1 + 10% waste), rounded up. Tiles per m² is the covering capacity on the data sheet of each of the 7 tiles in the list, at the batten gauge the sheet gives; where a sheet prints a second figure (a larger headlap, or a wider gauge allowed from a steeper pitch) the guides show both.
• Battens = roof area x (1000 / gauge in mm) x (1 + waste), rounded up to the metre: one batten every gauge up the slope.
• Ridge tiles = (ridge + hips) / 457 mm, the length of a Marley concrete ridge tile, rounded up.
• Tile cost = tiles x your price per tile. The supplied-and-fitted range = roof area x the £ per m² range for the material from the MyJobQuote cost guide.

No tile manufacturer we read publishes a waste figure, so the 10% starting value is IKO's rule of thumb for shingles, labelled as such wherever it appears. Calculator: roof tile calculator.

Units and rounding

The maths runs in metres and square metres and converts at the edge: 1 ft = 0.3048 m exactly, so one roofing square (100 sq ft) is 9.29 m². The region you pick changes the units, the currency and which cost guide is used, never the geometry. Quantities you buy are rounded up to whole units after the waste is added; areas and lengths are shown rounded for reading but carried at full precision.

Where the starting figures come from

Coverage figures come from manufacturer data sheets and specification guides first. Where a manufacturer does not print a figure (a price, the length of a drip edge piece), we use a named merchant listing and say so. Cost ranges come from published cost guides with their year. Rules of thumb are labelled as rules of thumb. Each calculator and each guide ends with a Sources list that quotes the exact line we used. The 21 documents behind the starting figures:

• Owens Corning, asphalt shingles specification guide (Section 07 31 13), 2013, read on 2026-09-20
• IKO, How Many Square Feet in a Bundle of Shingles, 2023, read on 2026-09-20
• IKO, Blueprint for Roofing (measuring and installation guide), read on 2026-09-20
• Atlas Roofing, synthetic underlayment product page, read on 2026-09-20
• GAF Seal-A-Ridge ridge cap, retail listing (Superarbor), read on 2026-09-20
• IKO, Hip & Ridge 12 technical data sheet, read on 2026-09-20
• Fastener USA, How to Calculate How Many Roofing Nails You Need, 2023, read on 2026-09-20
• Amerimax 10 ft aluminium drip edge, retail listing (Ace Hardware), read on 2026-09-20
• HomeAdvisor, How Much Does a Roof Replacement Cost?, 2026, read on 2026-09-20
• Marley, Modern concrete tile technical specification, read on 2026-09-20
• Marley Modern, merchant listing (Roofsy), read on 2026-09-20
• Marley, Mendip concrete tile technical specification, read on 2026-09-20
• MyJobQuote, Roof Tile Cost Guide, 2026, read on 2026-09-20
• Marley, Ludlow Plus concrete tile technical specification, read on 2026-09-20
• BMI Redland, Redland 49 product page, read on 2026-09-20
• BMI Redland, Grovebury product page, read on 2026-09-20
• Redland Grovebury, merchant listing (About Roofing Supplies), read on 2026-09-20
• Marley, Acme single camber clay plain tile technical specification, read on 2026-09-20
• Roofing Slate Direct, Required Slate Quantity, read on 2026-09-20
• SIGA Slate, How many slates do you need per m²?, read on 2026-09-20
• Marley, ridge and hip tiles, read on 2026-09-20

How often the figures are re-read

Every figure carries the day it was read; today the starting values were read on 2026-09-20. We re-read the cost guides when their publishers put out a new year's figures, and every data sheet at least once a year. When a figure changes, the date next to it changes, and so does the “Last updated” date of every guide that quotes it. Before anything is published, automated tests recompute key results that were worked out by hand from the data sheets (for example the bundles for a shed or the tiles for 100 m²), and the site is not built if the code disagrees.

What the results cannot tell you

• They are planning quantities, not a survey, a quote or a specification. A roofer measures the roof before ordering.
• Each section is a rectangle with one pitch. Valleys between sections, dormer cheeks, chimneys and skylights are approximated by the waste figure or by adding a section.
• The calculators do not cover flashing, valley metal, fascia, soffit, ventilation, the deck or the structure.
• Cost ranges are national averages from a cost guide; local prices, access, scaffolding and tear-off move a real quote well outside them.
• The data sheet of the product you actually buy, and your local building code, decide the minimum pitch, the gauge and the fixing. Nothing here is safety guidance for working on a roof.

Reporting an error

If a figure looks wrong, a source has changed, or a data sheet you have says something different, email hello@rooftally.xyz with the page address, the figure and, if you have it, the document that shows the right value. We check it against the source, correct the page if it is wrong, and change the dates. Corrections are free, and we reply to every report.

Who runs the site and how it is paid for: About RoofTally. All guides: roofing guides, for example how many roof tiles per m².

Last updated 2026-10-21.